Microstructurally controlled mullite ceramics produced from monophasic and diphasic sol-derived pastes using extrusion

Mullite ceramics with controlled microstructure in terms of grain size/shape, pore and glassy phase content were produced from sol-derived pastes using extrusion. Particular attention has been given to the development of a continuous process which is suitable for the preparation of high-solids-loading mullite pastes from two different starting mullite precursors, namely, diphasic and molecular mixed mullite sols. A combined processing technique comprising vacuum filtering and pressure filtration was introduced in order to obtain extrudable mullite pastes from low solids-loading colloidal sols. It is shown that glassy phase free stoichiometric 3:2 mullite (3Al2O3·2SiO2) with fine (0.94 μm) equiaxed grain microstructure is achievable from monophasic precursors after pressureless sintering at 1400°C for 3 h using the developed technique which can control both the sol-derived paste microstructure and process parameters. It is also found that the room and high temperature (1300°C) flexural strength and toughness of extruded mullites are mainly controlled by the grain size, the presence and location of glassy phase, nano-inclusions and pores at the grain boundaries. Pressureless sintered mullite derived from the monophasic sol-derived pastes provides flexural strength values of 345 and 277 M Pa for room temperature and 1300°C, respectively.